Integrated circuits having bipolar and MOS transistors formed on the same semiconductor substrate have many applications in the electronics industry and are therefore in great demand. They combine the high power and fast switching speeds of bipolar devices with the high density and low power consumption of MOS transistors.
When forming devices using a bipolar complementary metal oxide semiconductor (BiCMOS) manufacturing process, care is taken to minimize the number of masks employed therein to lower the manufacturing costs. Therefore efforts are made as often as is practicable to integrate the use of regions typically utilized for CMOS/DMOS devices as regions in a bipolar device, and vice-versa. In BCD (Bipolar-CMOS-DMOS) technology, bipolar devices are therefore usually “mask-free” since they do not make use of dedicated masks for the base, emitter, and collector, but make use of existing process layers. While such integration does serve to minimize manufacturing costs, in some cases the integration causes performance tradeoffs to be made.
For example, prior art FIG. 1 illustrates an NPN type bipolar transistor 10 fabricated using a BiCMOS type fabrication process. The transistor 10 has an n-buried layer (NBL) 12 that is formed in a lightly doped P-type substrate 14. A P-type epitaxial (Pepi) layer 16 is then grown over the NBL 12 and the substrate 14. A deep N+ ring 18 is formed by performing either an N-type implant or N-type thermal deposition in the epitaxial layer 16. The deep N+ ring 18 extends down to the NBL 12 to couple with the NBL 12 and define a collector region. The deep N+ ring 18 also defines therein an isolated base region 22 comprising the Pepi. The N+ region 18 is usually configured as a ring to provide isolation and serve as a plug extending down to the NBL region 12 for purposes of making contact thereto. A P-type source/drain implant is then performed to define a base contact region 24 and an N-type source/drain implant is performed to form an emitter region 26, wherein the base contact region is formed concurrently with the formation of PMOS source/drain regions elsewhere, and the emitter region is formed concurrently with NMOS source/drain regions elsewhere, respectively.
The NPN bipolar transistor 10 of prior art FIG. 1 may be employed in various types of applications, and in some applications the collector-to-emitter breakdown voltage (BVCEO) of the transistor 10 may be an issue.
Another consideration in bipolar transistor is its gain, which is sometimes referred to as the transistor β or HFE. When using the BiCMOS process described above, the spacing between the N-type source/drain region 26 which forms the emitter and the deep N+ ring 18, which forms the collector of the lateral NPN Bipolar transistor, is quite large, which contributes to poor bipolar transistor gain.
Therefore, there is a need in the art for a CMOS/DMOS manufacturing process that allows for optimization of bipolar transistor parameters, including parameters related to horizontal bipolar transistors, without significantly increasing the number of steps and/or masks required in the process.
FIG. 2 shows another prior art BiCMOS structure that defines a medium voltage NPN device. The emitter of the NPN bipolar transistor is defined by an n-type source-drain region (NSD) region 210. The base is formed by the p-epitaxial region (Pepi) 212 and a p-buried layer (PBLMV) 214. An n-buried layer (NBL) 216 with its DEEPN 218 formed in a deep trench region providing contact to the NBL 216 defines the collector of a vertical NPN transistor, while the shallow n-well (SNW) 222 with its n-type source-drain (NSD) contact region 224 defines the collector of a lateral NPN transistor. Current flows from emitter to collector both in vertical (NSD-PBLMV-NBL) and lateral (NSD-Pepi-SNW) directions, but lateral current prevails for typical device dimensions.
BVCEO of this device is limited by Pepi-SNW or Pepi-DEEPN junction breakdown and is often not high enough for device operation.